Projecting lens



I k n 0R 1,479,251 n qma y .CIADPU www# Jan. l, 1924 1,479,251

W. H. REPP PROJECTING LENS 7 Filed July s1. 1922 7' i; 5

`Rad a nammaw E 2 s w c PLANO *f Q1 Pugno nimm 7'" a 3-53 2.-

05 5 Screen W t nl x ii I n m 12 (A B Glas.: sid i Lens In III 'bwwo v--as n u n n n&]" "D aaa v z lNVENTOR William H. Re?? WHORE' EY Patented Jan. l, 1924.

UNITED STATES Draisman WILLIAM H. REPP, OF ROCHESTER, NEW YORK.

PROJ ECTING LENS.

Application led July 31.

To all whom it may concem.'

Be it known that I, WILLIAM H. citizen of the United States, residing at Rochester, in the county of Monroe and State of New York, have invented certain new and useful Improvements in Projecting Lenses, of which the following is a specilication.

The object of my present invention is to provide a new type of projecting lens which is especially adaptable for use in moving picture machines.

Another object of this invention is to simplify the construction of such a lens by reducing the number of free surfaces thereof and designing them in a manner that will permit an easy cleaning of the elements of the lens.

Another object of this invention is to mount and design the elements of the lens so that after the lens has been taken apart for cleaning these elements can be quickly and easily replaced in the order of the formula.

Another object of this invention is to make the curves of the elements from which the lens is built up shallow to increase the transmission of light through the lens and also materially reduce the cost of manufacture of the lens.

These and other objects of this invention will be fully illustrated in the drawing, described in the specification and pointed out in the claims at the end thereof.

In the accompanying drawing.

The ligure is a diagrammatic view of my improved projection lens.

In the projecting lenses based on the Petzval formula, on a modified and improved form of which Patent N o. 1,415,002 was granted to me May 2, 1922, the elements of the rear combination are kept apart by a spacing ring. This practice has been adhered to in the construction of cino lenses because it has always been feared that the heat rays caused by the light source of the proj ection machine would be detrimental to a. cemented combination of these elements of the lens, especially as they were located nearest the heat or light source. However, it is my opinion that if properly taken care of the rear combination of the elements of a projecting lens can be cemented with practically as much immunity against the blistering and melting of the cement as the elements of the front combination which are cemented at the present time and considered good practice;

Rnrr, a

1922. Serial No. 578,695.

this in spite of the fact that blisters and melting of the cement are caused in the front combination when the system is left unprotected for a considerable length of time as shown in many instances. The danger of blistering and melting may be slightly greater in the rear combination than it is in the front system, but the advance in the art of projecting apparatus and the skill in which these are operated by competent projectionists who know how to handle high grade lenses and take care of them almost eliminates this danger and need not be feared now as much as when the art was in its infancy.

I, therefore, believe that my present invention in which both the front and rear consist of cemented doublets are highl practicable in addition to being more eii-1 cient on account of the elimination of the loss of light caused by refiection due to the separation of the elements in the rear combination in the old systems of projecting lenses.

For example, without considering the iniiuence of the cemented surfaces which is practically nil, the transmission of light through a miscellaneous system of cemented lenses may be estimated roughly using the well known expression of F resnel for the loss of light (L) at each surface of the refractive index.

Assuming an average index of 1.56 we find that a single lens system containing two free surface transmits 90% of the incident light; a two lens system containing four free surfaces transmits 82% a three lens System containing six free surfaces transmits but 7 4% of the incident light, so that, therefore, in cementing the rear combination of my projecting lens the free surfaces thereof are reduced from six to four with a corresponding increase in the brightness of the image of about one-tenth. y

In addition to the increase in the transmited light there is a reduction of the number of secondary reflector images. Thus, a lens system containing a number of (m) free surface produces secondary images so that a single lens gives one secondary image, a two lens system gives six secondary images and a three lens system gives fifteen secondary images. This means that in my system of two cemented lenses only the formation of six secondary images takes place. The reduction of these secondary images from fifteen to six, when compared with my modified Petzval objective, adds, therefore, to the brilliancy of the screen image.

A big and import-ant factor in the use of projecting lenses for moving picture machines is the simplicity of the construction of the system of the objective. All lenses used in moving picture machines should be cleaned at least twice a week and not only on the outer surfaces thereof but also the inner surfaces of each of the elements. This is a necessary evil as it does not improve the quality of the delicate lens surfaces and the less surfaces need cleaning the longer the usefulness of the objective will be. In cleaning a projection lens there also enters the danger of misplacing the elements of the lens when replacing these elements after having been cleaned. As these elements must be replaced according to a formula, any misplaced elements result afterwards in unsatisfactory projection. A common error made is that the crown is reversed in its position, that is, with the strong radius outside but even the front combination is sometimes placed in the place of the rear combination in the Petzval type of proj ectingl lens.

In the practice of my invention, as illustrated diagrammatically in the figure of the drawing, I aim to produce a compound unsymmetrical optical system, corrected for colors, spherical aberrations, coma, and fiatness of field which can be used for a very high aperture ratio (-F 2.2). In the manufacture of this system I employ besides plano surfaces or a basic weak radius which are common to all focal lengths only two different radii for each focal length. These are employed in the front system A and rear system B respectively and produce an unsymmetrical construction of which each can be cemented if preferred. The positive elements of each system are double or equiconvex and the negative elements are plano concave.

In this way the number of shapes of elements are reduced when compared with vthe Petzval objective from four to but two by eliminating the strong negative meniscus a lens which is both troublesome and expensive to make as well as the plano convex element. This necessarily also reduces the number of blanks and molds for making this lens with a resulting saving in the manufacture thereof.

In simplifying the objective by making the rear combination in one unit as above pointed out the operator of the moving picture machine can easily remember this unit by its smaller size and stronger curve and after having cleaned all of the four surfaces of the objective will not make a mistake in replacing this rear combination in the wrong position.

The diagrammatic illustration of the figure of the drawing represents a full size of the lens system embodying my invention having an equivalent focal length (E. F.) of 100 in/m and a relative aperture ratio of F/2.2. From this illustration it will be seen that the thickness of the respective crown and flints of the front and rear system are practically the same so that the same blanks and molds can be used for them.

The front combination which is cemented consists of an equiconvex lens I made of crown of low index which faces a plano conV cave lens II made from a flint of high index. This combination although corrected for color and spherical aberrations covers if used alone only a small central part of the eld satisfactorily, its main defects being coma and curvature of field. The focal length of this combination is almost twice as long as the E. F. of the whole system with the result that the radii are very shallow. These curvatures when compared with those in the improved Petzval objective illustrated in my prior patent above referred to are 1.45 longer.

The rear combination which is preferably also cemented although it may be made with a thin spacing ring is an exact reproduction of the front system on the scale 2:1 except for the thickness of the elements which are practically the same. This proportion of 2:1 of the lenses in the front and rear combination need not be strictly adhered to. As illustrated the radii of the rear combination however, are one half of the radii of the front combination and its focal length is, therefore, nearly the E. F. of the system. Thus, the rear system itself is affected by the same faults enumerated above in connection with that of the front system. However, both of the systems combined as specified by the formula make u a system which is remarkabl free from al defects lowering the quality of the screen image. The separation or interval between the front and rear halves of the system is approximately equal to the E. F. of the system with slight variations either up or down as may be required when the objective is applied to a special type of machine to meet the special requirements calling for a longer or shorter over length.

Such slight changes in this distance may also require slightly different curves in either the front or rear combination but such variations are understood to be within the scope of my invention.

A comparison of my new type of projection lens with the lens illustrated in my llO Patent No. 1,415,002 will show that the strongest radius in the latter is 38.314 m/m, while in my present invention the strongest curvature is 41.28 m/m. This, however, is not the limit and can be increased by weakening the curvature of the rear system especiall if a slightly stronger curvature of the fie d can be tolerated. This is the case when the system is adopted for longer focal len hs.

hile my invention has been worked out primarily for use in movin picture projection it is by no means limited to this field but can be utilized in other elds of applied optics such a telescopic objectives, photographic lenses, etc.

Referring again to the figure of the drawing, an example is submitted by means of which a comparison as regards shapes of lenses curvatures and separat-ions can be made between my present invention and other types of projection lenses such as the lens illustrated in my prior patent above referred to which will enable any one skilled in the optical design to practice my invention.

The simple proportion 2 :1 :1 :1 which exist between the front the rear, the separation and the E. F. of the whole lens system is illustrated because it enables the assembler to pick his rear element measure its focal length, find another doublet of twice the focal length and mount them with a separation equal to the shorter doublet. In view of the fact that on account of the variation of the index in different melts the focal lengths of the elements are subjected to constant small changes this simple procedure in the assembly of the lens system is of great advantage.

The example is worked out for an E. F. of 100 m/m and a free aperture of the front system of 45.4 m/m therefore for a relative aperture ratio of 100; 45.4 or F/2.2. The screen intercepting the magnified image is supposed to be at the left and the film to be projected at the bottom of the lens system at a distance slightly longer than 39.82 m/m from the last plano surface. The number of the lenses from left to right are indicated by I, II, III and IV and their radii will be indicated by reference characters 11, 11 r3, and r4 respectively.

The axial thickness will be indicated by reference characters d, and d2 and the spacing distance between the two systems by reference character A. The glasses are specified by nD and V where nD means the refractive index for the yellow line in the solar spectrum and the value V (the dispersive reciprocal) as defined by Abbe:

Draftsman glasses for the C line (light red) and F ine (greenish blue) in the spectrum.

From the foregoing it will be seen that only two different curves are employed in this system of lenses in spite of the fact that only equiconvex and planoconcave elements are used to make up the objective and with this combination of elements I secure an unsymmetrical construction of a very high aperture possessing all the qualities such as freedom of colors, perfect central definition and flat field, which form the prerequisites of a good projection lens, this with a material reduction in the cost of manufacture of the lens.

I claim:

l. A high aperture projection lens system, comprising a pair of doublets made up of an equi-convex and a plano concave lens, the equivalent focus of the front doublet bein twice the equivalent focus of the rear dou let, said doublets bein separated by a distance equal to the equivalent focus of the complete lens system the convex sides of said doublets facing the screen and the plano surfaces of said doublets facing the object to be projected on the screen.

2. A high aperture projection lens system, comprising a pair of doublets, the equivalent focus of the front doublet being twice the equivalent focus of the rear doublet, said doublets being separated by a distance equal to the equlvalent focus of the complete lens system, each of said doublets comprising a double convex lens made from low index crown glass and a plano concave lens made from high index flint glass.

3. A high aperture projection lens system, comprising a pair of doublets, the equivalent focus of the front doublet being twice the equivalent focus of the rear doublet, said doublets being separated by a distance equalto the equivalent focus of the complete lens system, each of said doublets comprising a double convex lens made from low index crown glass and a plano concave lens made from high index flint glass, each of said double convex lenses of said doublets facing the plano concave lens of the same doublet and being cemented thereto.

4. A high aperture projection lens system of unsymmetrical construction comprising a pair of doublets, said doublets being made up of lenses having but two different curvatures.

5. In a high aperture projection lens system, the combination of a pair of doublets made up of an equi-Convex and a plano concave lens, the convex sides of said doublets facing the screen and the plano surfaces of said doublets facing the object 10 to be projected on said screen by said projection lens system.

In testimony whereof I aix In si ature.

WILLIAM R PP. 

